This invention relates to graphical data apparatus and, more particularly, to an apparatus for locating the position of a movable element in a data space.
Graphical data systems have been employed for some time to determine the position of a movable element or stylus in a defined coordinate system. Earlier systems employed a stylus in the form of a writing pen or pointer mechanically coupled to a set of arms for translating the movement of the stylus into a sequence of usable information signals. Such arrangements proved generally unsatisfactory in that they presented undesirable frictional and inertial limitations.
Subsequently developed graphical data systems, called "digitizers," utilize tablets which have imbedded conductors. These systems, which involve the considerable expense of tablet manufacture, are satisfactory for some uses but are inflexible in that they are limited to applications where a tablet can be employed.
In the U.S. Pat. Nos. 3,838,212 and 3,731,273 there are disclosed graphical data systems which utilize a stylus that generates periodic sound waves and microphones which define a coordinate area. Circuitry is provided which measures the transit time of the sound waves between the stylus and the microphones, and the cooridinates of the stylus are determined from the transit times which are translatable to distances since the speed of sound in air is essentially a known quantity. These types of systems are presently in commercial use and offer a distinct advantage over conventional data tablets since no tablet is required and the microphones can be positioned around any desired work space. Operation is considered satisfactory but there are aspects of these types of systems which could stand improvement. For example, presently employed microphones are generally of the capacitive type which require precise uniform layered structures that are generally difficult to manufacture and are relatively fragile. When electret layers are utilized the microphones are found to have a limited life, due to leakage of trapped charge, beyond which performance can degrade. Also, the stylus in such systems is active and generally requires wire connections for energizing the sound source. In addition to the effects of the wire on manual dexterity, the sound source, typically a relatively high voltage spark, can cause electrical interference in some instances; e.g., where the data area corresponds to an electronic display like a CRT or a gas discharge display.
It is an object of the invention to provide a graphical data system which overcomes disadvantages of the prior art as set forth.